When buildings are being erected, it is often necessary or desirable to complete an excavation so that a more stable earth structure for foundations can be reached or to provide basement levels for the building. To prevent cave-ins of the surrounding earth into the excavated area, shoring is placed at a position that will be outside the intended basement walls. The requirement for a structure adequate to withstand the loads that may be imposed while providing a relatively uncluttered work area has previously led to the provision of shoring systems that are supported at least in part by tie-backs or earth anchors. Necessarily, the loadings to which the shoring will be exposed and also the holding capability of various anchor systems is largely dependent on the type of earth structure that is to be excavated and full consideration must be given both to the characteristics of the earth structure where the anchors are to be set and also to the characteristics of the overburden thereabove. If rock or other heavily consolidated structures are involved at the levels to be excavated or at positions closely therebelow, the anchoring techniques used can be relatively simplified. In some instances the tie-backs or tendons are simply extended into the undisturbed rock structure and an anchor end or bushing for the tendon is grouted in place.
A related but more severe problem is encountered where anchors must be established in a more loosely consolidated earth structure. Under such conditions the tension loadings to be established in the tendons must be transferred to the earth structure in a manner that will allow distribution of such loadings efficiently into a larger bulk area or volumetric zone of the supporting earth structure. The problem is further complicated by the fact that the loadings tending to collapse the shoring system are increased when a loosely consolidated overburden type of material is being excavated.
Tie-backs, anchor and tendon installations for use in relatively loose soils are known in the shoring system field, and it is acknowledged that others have previously been concerned with the provision of land anchors for other tie down purposes. Land anchors used for tie down purposes in connection with the anchoring of building structures, vehicles, fences, utilities, etc. are not believed to be directly comparable especially if the anchors are driven from an above-ground site. For such installations the loadings are generally less than the strength of the main force distributing anchor element itself. One earlier type of anchor used for a comparable purpose is illustrated and described in an earlier U.S. Pat. No. 3,728,862 granted to the present inventor. In such earlier disclosure an anchor plate is used that is inserted with a minimum frontal area, and it is later turned to provide a maximum frontal area exposed to the supporting earth structure. Another tie-back for use in connection with excavations adjacent to rock type structures is shown in a patent to R. E. White, U.S. Pat. No. 3,226,933. The total loadings to be developed through use of the present invention are comparable to those contemplated in the White disclosure, but the method and means used herein for attainment of the desired anchor capability is believed to be at least as different from the earlier patent disclosure as the respective soil structures that are to be held.